This invention relates to a motion-vector detection apparatus for an input video signal, particularly to a motion-vector detection apparatus used in a motion compensated interframe coding apparatus for the input video signal.
In the motion compensated interframe coding, a frame of the input video signal is divided into a plurality of blocks each consisting of, for example, 8 scanning lines.times.8 picture elements. Then, a motion vector of each block is detected on the basis of evaluation function values, which indicate positional dissimilarity between the block of a present frame and a two-dimensionally shifted block of a previous frame. One of the shift vectors representing the minimum evaluation function value is detected as a candidate motion vector of each block of the present frame.
Referring to FIG. 1, a conventional motion-vector detection apparatus is hereinafter described.
As shown in FIG. 1 the conventional motion-vector detection apparatus comprises:
a motion-vector detector (MV-DET) 3 for detecting and outputting the candidate motion vector of the present frame 36 (V'.sub.opt x, V'.sub.opt y) and the minimum evaluation function value 35 (.SIGMA..vertline.MCFD.vertline.) corresponding to the candidate motion vector; PA1 a frame difference accumulator (FDA) 2 for accumulating absolute values of frame differences in each block between a present frame video signal 21 and a previous frame video signal 22 and outputting the accumulated value 25 (.SIGMA..vertline.FD.vertline.), which corresponds to an evaluation function value of a zero shift vector; PA1 a comparison section 5 for comparing a difference between the minimum evaluation function value 35 (.SIGMA..vertline.MCFD.vertline.) and the accumulated value 25 (.SIGMA..vertline.FD.vertline.) with a predetermined threshold (T) and generating a vector selection control signal 56; and PA1 a selector 6 for selecting one of the candidate motion vector 36 and a zero vector in accordance with the vector selection control signal 56 and outputting the selected one as a motion vector 61 (V'.sub.opt x', V'.sub.opt y). PA1 a frame difference accumulator for accumulating the absolute values of frame differences in each block between the present frame and the previous frame of the input video signal and outputting the accumulated value in each block; PA1 a motion-vector detector for detecting a candidate motion vector of each block and for outputting the candidate motion vector and an evaluation function value of the candidate motion vector; PA1 a region detector for classifying blocks of the present frame into stationary parts and motion parts and for outputting a threshold selection control signal; PA1 a first selector for selecting one of at least a first threshold and a second threshold in accordance with the threshold selection control signal and outputting the selected threshold; PA1 a comparison section for comparing the difference between the accumulated value and the evaluation function value with the selected threshold and for outputting a vector selection control signal; PA1 a second selector for selecting one of a zero vector and the candidate motion vector in accordance with the vector selection control signal and outputting the selected one in each block as the motion vectors of the input video signal.
The comparison section 5 includes a subtracter 51 for subtracting the minimum evaluation function value 35 (.SIGMA..vertline.MCFD.vertline.) from the accumulated value 25 (.SIGMA..vertline.FD.vertline.); an absolute value calculating circuit 52 for calculating an absolute value of an output of the subtracter 51; and a comparator 53 for comparing the absolute value with the threshold (T) and supplying the vector selection control signal 56 to the selector 6.
The operation of the conventional motion-vector detection apparatus will now be explained.
The frame difference accumulator 2 accumulates in each block the absolute values of the frame difference signal between the present frame video signal and the previous frame video signal. Then, the frame difference accumulator 2 outputs the accumulated value 25 (.SIGMA..vertline.FD.vertline.) to the comparison section 5.
The motion-vector detector 3 calculates evaluation function values in each block of the present frame for various shift vectors and detects the minimum value among them. Then, the detector 3 outputs a shift vector corresponding to the minimum value as the candidate motion vector 36, which is supplied to the selector 6. In addition, the detector 3 outputs the minimum value 35 (.SIGMA..vertline.MCFD.vertline.) to the comparison section 5.
In the comparison section 5, the subtracter 51 subtracts the minimum evaluation function value 35 (.SIGMA..vertline.MCFD.vertline.) from the accumulated value 25 (.SIGMA..vertline.FD.vertline.) and outputs the subtracted value of each block to the absolute value calculating circuit 52. The absolute value calculating circuit 52 outputs an absolute value D of the subtracted value, or D=.vertline..SIGMA..vertline.FD.vertline.-.SIGMA..vertline.MCFD.vertline.. vertline. to the comparator 53.
The comparator 53 compares the value (D) with the predetermined threshold (T). If the value (D)is larger than the threshold (T), the comparator 53 outputs a first level of the vector selection control signal 56, which causes the selector 6 to select the candidate motion vector 36 as the motion vector 61 of each block. Otherwise, the comparator 53 outputs a second level of the selection control signal 56, which causes the selector 6 to select the zero vector as the motion vector 61 of each block.
In other words, the comparison section 5 causes replacement of the candidate motion vector 36 of each block with the zero vector when the difference between the minimum evaluation function value (.SIGMA..vertline.MCFD.vertline.) and the accumulated value (.SIGMA..vertline.FD.vertline.) is small. This prior art is disclosed in Japanese Laid-Open Patent Publ. No. 58-107785 (1983).
The replacement of the candidate motion vector provides better reproduced picture quality in a stationary uniform background part of a reproduced video signal. If the motion compensated interframe coding without the replacement is applied to the stationary uniform background part, there is a case where non-zero vectors are detected as the motion vectors in the background part due to noises included in the input video signal. In this case, noises due to the coding (referred to hereinafter as "coding noises") are observed to move in accordance with misdetected motion vectors due to the noises in the input video signal. The coding noises which move in the stationary uniform background provide observers with degraded picture quality. Since the difference between the minimum evaluation function value (.SIGMA..vertline.MCFD.vertline.) and the accumulated value (.SIGMA..vertline.FD.vertline.) is often small in the stationary uniform background part, the modification improves the picture quality of the reproduced video signal.
However, the effect of improving the picture quality by this replacement is insufficient because an optimum value of the threshold (T)is very sensitive.
For example, FIG. 2A shows an example of motion-vector detection results by the conventional apparatus. In FIG. 2A, arrows show the detected motion vectors. One can see that the non-zero candidate motion vectors are insufficiently converted to zero vectors.
It is possible to eliminate the misdetected candidate motion vectors completely by making the threshold (T) high. However, in doing so, the coding efficiency of the motion compensated interframe coding degrades because many candidate motion vectors in the motion part of the video signal are also converted to zero vectors as shown in FIG. 2B. On the other hand, if the threshold is made low, the number of the misdetected vectors in the stationary uniform background increases as shown in FIG. 2C.